Method of preparing uranium for nickel plating



United States Patent 3,275,535 METHUD 0F PREPARING URANIUM FUR NlCKlEL PLATING Jerry R. Lundquist and Robert W. Strornatt, Richland,

WaslL, assignors to the United States of America as represented by the United States Atomic Energy Corn- No Drawing. Filed Mar. 22, 1966, Ser. No. 538,157

3 Claims. (Cl. 204-15) The invention herein described was made in the course of or under a contract with the United States Atomic Energy Commission.

Our invention relates to a method of preparing uranium for nickel plating. The method comprises anodizing the uranium in an acid bath 5 to 7 M in H 80 and 0.25 to 0.40 M in HCl.

The object of the invention is to prepare uranium slugs for fabricating nuclear fuel elements by what is known as the hot die sizing process, which is an application of the process of US. Patent No. 2,848,800, granted August 26, 1958, to James O. Maloney, Earl B. I-Iaines and John B. Tepe.

In the hot die sizing process, nickel plated uranium slugs are placed in aluminum cans and the assembly is then heated and forced through dies, as shown in the Maloney et a1. patent. The nickel plate acts as a diffusion Step Degrease:

Rinse Pickle...

3,275,535 Patented Sept. 27, 1966 which the uranium is pretreated and plated react with uranium, it is not possible to prepare a surface completely oxide free, and a compromise must be reached. For the resulting plate to be satisfactorily adherent and nonporous, the following criteria in the surface prepartion must be met. The oxide film on the uranium after pretreatment should be uniform to allow a uniform plating current density, should provide some protection from the plating solution, and should not be bulky or passive in nature. The surface should have a uniform etch to allow uniform plating current density and provide sufficient lock and key adherence between the uranium and the nickel, but should not have deep valleys or high peaks which can cause non-contact and thin spots, respectively. For economic reasons the amount of uranium removed in these steps should be minimal.

Anodization of uranium in a solution of H SO HCl after suitable degreasing and pickling steps, and followed by an HNO pickling step, provides a uranium surface which satisfies the above criteria. Table I gives the conditions for the procedure to prepare a uranium sample for plating. The anodic uranium product is not very soluble in the H SO -HCl, and it is easily removed from the solution since it is not the gelatinous precipitate often obtained in anodization techniques.

TABLE I electroplating and secondary corrosion barrier, prevents the formation of undesirable aluminum-uranium compounds, and the uranium-nickel-aluminum bond formed in the cladding operation provides good heat transfer. In order that a satisfactory bond may be formed, it is necessary that the nickel plating be nonporous and that it adhere to the uranium. It is desirable that the nickel be as thin as possible, to keep parasitic neutron capture to a minimum.

We have found that the anodization treatment mentioned above produces superior results, as compared to known treatments, when the uranium is subsequently electroplated. Non-porous coatings can be obtained with considerably less nickel thickness. Moreover, less uranium is removed than in prior methods.

As received, uranium fuel elements cannot be directly nickel plated, but first must be cleaned. To obtain a suitable surface, not only foreign matter and uranium oxides, but also some of the work-distorted uranium crystals must be removed. Since the condition of the resulting uranium surface has a significant effect on the quality of the plate, the surface preparation is very important. Common methods for surface preparation involve chemical, mechanical, or electrolytic etching operations. Several such techniques are: Sncl -ethanol and aqueous CuCl HF chemical etches; shot blasting mechanical etch; and anodization in solutions of H PO HCl, CCl COOHHC-1, CH COOHHCl, HNO and others. Most of these etchants require an HNO pickling step to remove a black reaction product from the uranium surface.

Since the atmospheric and aqueous environments in Current Density, ma. lcrn.

Time,

Acetone 0.1% Duponol in 2M NaOH Demineralized H2O dip 10M HNOs Running demineralized H20 5120 7M I'IzSOq 0.25 to 0.40M HCl Running demineralized H O.

O3 Running demineralized H1O Ambient.... 1

Ambient 40 5.- Ambient Numerous experiments have demonstrated that best results for the fuel elements are obtained under the following conditions: 6.0 M H SO 0.35 M HCl, 59 milliamperes per square centimeter, 4 minutes, and ambient temperature (about 25 0). Samples pretreated in the fashion were plated using Watts, and three sulfamate, nickel plating solutions. Nickel plating bath compositions and temperatures and current densities used are listed below:

TABLE II Current Density, Ina/em.

Temp- Plating Bath Composition eratlre, pH

0.20M NlBl'z 0.62M HaB O3 0.5% XXX-D (wetting agent) Sultamate (Barrett):

1.4M Ni (NH2S03)2 0.02M NlCl2.6H2O 0.97M HsBOa Suliamate:

1.4.1 Ni (NHzSOa)z 0.97M HsBOs 0.5% XXX-D (wetting agent) Sulfarnate (Hanson-Van-Winkle- 0.73M HrBOa 4.2% SBSN-l (brightener 0.14% SBSN-2 (brightener).-. 0.12% Anti Pit 7 (wetting agent) We found that the number of pores could be decreased by a factor of about three by substituting bromide for chloride in the Watts plating solution. The plating obtained in the Watts bromide and the various sulfamate solutions was about the same in terms of porosity and integrity. The samples were tested for porosity and adherence by an X-ray technique and by inspection after maintaining them at 600 C. in the air for five minutes followed by air cooling.

Adherence and porosity of these samples were significantly improved over samples prepared by other similar techniques. For example, an 11 micron nickel plate on samples anodized in the H SO HCI solution was equivalent in terms of adherence and porosity to 18 microns nickel on samples prepared by anodization in the H PO.,HCI solution. For plates of equal thickness, the number of pores is decreased by a factor of about 8 using the H SO HCI solution rather than H PO HCl.

This improvement is believed to be due to a difference in roughness between the etched surfaces. For example, a considerable decrease in surface roughness was obtained by substituting H SO HCl for H PO HCI. A rough estimate of the difference in roughness (or surface area) was obtained by double layer capacitance measurements, the measured capacity being proportional to the surface area. The double layer capacity for uranium anodized in 5.4 M H PO 0.26 M HCl was 57 tf./cm. and in 6.0 M H SO 0.32 M HCl, 48 ,uf/CIIL a decrease of about 20%.

In addition, use of this anodization method results in a 60 percent decrease in the uranium removal without sacrificing plate quality. Satisfactory results were obtained after removing 14 milligrams per square centimeter, while 30 to 150 milligrams per square centimeter should be removed using various other etching techniques.

An evaluation was made of the eifect of the H SO.,HCl anodic etch pretreatment upon the behavior of the nickel cladding in the hot die sizing operation. Three test cylinders were prepared as follows:

(1) Cylindrical sections of standard-quality ingot uranium, ca. 1 /2" long, were pretreated with either the standard H PO HCI anodic etch procedure or the H SO HCl anodic etch test procedure.

(2) The sections were plated with nickel, an 18 1. coat being applied to the standard specimens and a 913,u coat being put on the test specimens.

(3) Cylindrical assemblies, approximating the geome try of a standard production reactor fuel element of the type shown in the Maloney et al. patent, were then prepared, each cylinder containing three standard seg- 5 ments and two test segments.

(4) Each assembly was clad with an 8001 alloy aluminum cap and can using the hot die sizing technique.

For evaluation, four diameter aluminum studs were welded to and then pulled from each 1 /2" long segment. The bond strengths for test and standard material were found to be about the same.

While we have described processes in considerable detail it will be apparent that various changes are possible. We, therefore, wish our invention to be limited solely by the scope of the appended claims.

The embodiments of the invention in which an exclusive right or privilege is claimed are defined as follows:

1. A method of preparing uranium for electro-plating which comprises anodizing the uranium in an aqueous solution which is 5 to 7 M H 80 and 0.25 to 0.40 M I-ICl.

2. A method as described in claim 1 in which the temperature is 25 to 50 degrees centigrade, the current density is 30 to 90 milliamperes per square centimeter and the time of treatment is 3 to 10 minutes.

3. A method as described in claim 1 in which the solution is 6.0 M H 50 0.24 M HCl, the current density is 59 milliamperes per square centimeter, the treatment time is four minutes, and the temperature is about 25 degrees centigrade.

References Cited by the Examiner UNITED STATES PATENTS 2,854,738 10/1958 Gray 204-1.5 X 2,894,883 7/1959 Walker et al. 2041.5

References Cited by the Applicant UNITED STATES PATENTS 2,872,389 2/1959 Slunder.

OTHER REFERENCES Euratom Report EUR-17.1 F. Brossa et al., Study of Nickel as a Diffusion Barrier Between Uranium and Aluminum.

REUBEN EPSTEIN, Primary Examiner. 

1. A METHOD OF PREPARING URANIUM FOR ELECTRO-PLATING WHICH COMPRISES ANODIZING THE URANIUM IN AN AQUEOUS SOLUTION WHICH IS 5 TO 7 MH2SO4 AND 0.25 TO 0.40 MHCL. 